The present invention relates to diene rubber compositions usable for the manufacture of tires or semi-finished products for tires, in particular of treads for these tires, and to reinforcing fillers capable of reinforcing such rubber compositions.
So as to reduce fuel consumption and the pollution emitted by motor vehicles, major attempts have been made by tire designers to obtain tires having all of the following: very low rolling resistance, improved adhesion both on dry ground and on wet or snow-covered ground, and very good wear resistance.
Numerous solutions have thus been proposed to lower the rolling resistance and to improve the adhesion of tires, but these generally result in a very great decline in the wear resistance. In particular, it is well known that although the incorporation of conventional white fillers such as, for example, conventional silicas or aluminas, chalk, talc, clays such as bentonite or kaolin, in rubber compositions used for the manufacture of tires and in particular of treads, does result in a reduction in the rolling resistance and an improvement in adhesion to wet, snow-covered or icy ground, it also results in an unacceptable decline in the wear resistance, linked to the fact that these conventional white fillers do not have a sufficient reinforcement ability with respect to such rubber compositions. For this reason, these white fillers are generally referred to as non-reinforcing fillers, also referred to as inert fillers.
One effective solution to this problem has been described in particular in patent applications EP-A-0501227, EP-A-0735088 or WO99/02602, which disclose diene rubber compositions reinforced with precipitated silicas of high dispersibility (so-called “HD” silicas for “highly dispersible silica”), which make it possible to manufacture treads having a significantly improved rolling resistance, without adversely affecting the other properties, in particular those of grip, endurance and in particular wear resistance. Other compositions having such a compromise of contradictory properties are also described in applications EP-A-0 810 258 and WO99/02602, with specific aluminous fillers of high dispersibility as reinforcing white fillers.
However, the rubber compositions that contain these specific white fillers, even when they are of a highly dispersible type, remain more difficult to work (“processability”) than rubber compositions filled conventionally with carbon black, because for reasons of mutual attraction these white filler particles have an irritating tendency to agglomerate together within the elastomeric matrix. These interactions tend to increase the consistency of the compositions in the uncured state and therefore to make them more difficult to work compared with conventional compositions filled with carbon black; they also have the consequence of limiting the dispersion of the filler and hence the reinforcing properties to a substantially lower level than that which it would be theoretically possible to achieve if all the (white filler/elastomer) bonds that could be created during the thermomechanical kneading operations were in fact obtained.
Furthermore, these inorganic fillers of siliceous or aluminous type also have the known drawback of significantly disrupting the vulcanization kinetics of the elastomeric compositions, compared with conventional compositions filled with carbon black. In particular, in the case of silicas, it is known that the resulting longer curing times may adversely affect the industrial processing of the elastomeric compositions, like that of the rubber articles containing them.
Now, the Applicants have discovered during their research that there are specific inorganic fillers, other than those mentioned above, which can also be used in the rubber compositions as true reinforcing fillers, that is to say, are capable of replacing conventional carbon blacks for tires. Unexpectedly, these specific inorganic fillers offer not only excellent processing ability to the rubber compositions containing them, but also very great dispersibility, both similar to those available with carbon blacks. They furthermore make it possible to overcome the aforementioned drawback relating to the curing kinetics, specific to the reinforcing white fillers such as silicas or aluminas.
Consequently, a first subject of the invention relates to a rubber composition based on at least (i) a diene elastomer, (ii) a reinforcing inorganic filler, (iii) a coupling agent providing the bond between the reinforcing filler and the elastomer, characterized in that said inorganic filler comprises a silicon carbide (hereafter referred to as “reinforcing silicon carbide”) having the following characteristics:                (a) a BET specific surface area of between 20 and 200 m2/g;        (b) an average particle size (by mass), (dW), of between 10 and 350 nm.        
It is known that, conventionally, silicon carbides have been used essentially in treads of tires to improve the grip on snow-covered or icy ground (see for example FR-A-655 105, FR-A-2 218 209, FR-A-2 293 325 or DE-A-2 457 446, DE-A-2 355 466, DE-A-2 457 446, DE-A3 218 124, EP-A-0 442 155, EP-A-0 885 925, JP-A-1985/258235, JP-A-1987/091304, JP-A1988/151506, JP-A-1990/091137, JP-A-1990/135241, JP-A-1990/266704, JP-A-1991/252432 or U.S. Pat. Nos. 5,162,395, US-A-3,878,147, US-A-5,223,333, US-A-5,733,650). In all these documents, the particles of silicon carbide, which are chosen for their coarse size and their very great hardness, have the function not of reinforcement, but of improving the grip by a well-known “claw” effect on snow or ice. Furthermore, it is essential to note that these conventional silicon carbides, which are generally present in a very small proportion, are always added to rubber compositions which furthermore contain a true reinforcing filler such as carbon black and/or silica.
The subject of the invention is also a process for obtaining a rubber composition usable for the manufacture of tires, in which there are incorporated in at least one diene elastomer, at least a reinforcing inorganic filler and a coupling agent providing the bond between the reinforcing inorganic filler and the elastomer, this process being characterized in that said inorganic filler comprises a silicon carbide having the following characteristics:                (a) a BET specific surface area of between 20 and 200 m2/g;        (b) an average particle size (by mass), dW, of between 10 and 350 nm,and in that the entire mixture is kneaded thermomechanically, in one or more stages, until a maximum temperature of between 110° C. and 190° C. is reached.        
Another subject of the invention is the use of a rubber composition according to the invention for the manufacture of rubber articles, in particular tires or semi-finished rubber products intended for such tires, these semi-finished articles being selected in particular from among the group comprising treads, underlayers intended for example to be positioned beneath these treads, crown plies, sidewalls, carcass plies, beads, protectors, inner tubes and airtight internal rubbers for tubeless tires.
The composition according to the invention is particularly suited to the manufacture of treads for tires intended to be fitted on passenger vehicles, vans, 4×4 vehicles (having 4 driving wheels), two-wheeled vehicles, “heavy vehicles” (that is to say subway trains, buses, road transport machinery (lorries, tractors, trailers), off-road vehicles), aircraft, construction, agricultural or handling machinery, these treads being able to be used in the manufacture of new tires or for recapping worn tires.
The subject of the invention is also these tires and these semi-finished rubber products themselves, in particular these treads, when they comprise a rubber composition according to the invention.
Another subject of the invention is the use, as reinforcing filler, of a reinforcing silicon carbide as defined above in a diene rubber composition.
Finally, another subject of the invention is a process for reinforcing a diene rubber composition, characterized in that a reinforcing silicon carbide as defined above is incorporated in this composition in the uncured state by thermomechanical kneading.